Often it is desired, i.a., to reduce the transmission of heat and noise through sheet metal air conveying ducts or other structures to conserve energy, to maintain the conveyed air within a specified temperature range, to prevent moisture from condensing on surfaces, and/or to reduce offensive noise. One technique for reducing heat and noise transmission involves applying an appropriate insulating material to a surface of a wall of the structure. The insulation can be placed on an outside wall, but most effective sound dampening is usually achieved by insulating the interior wall, especially in air conveying ducts in heating, ventilation and air conditioning systems. Frequently system designs also call for installing thermal insulation inside ducts.
Acoustical and thermal insulation is typically made of generally flat, glass fiber nonwoven webs formed into batts or continuous rolls. Rigid and semi-rigid sheet of insulation board are also utilized for insulation purposes.
It is sometimes desirable to laminate one or both faces of the insulation with a skin of foil, mesh, paper, film and the like. This provides a surface for conveniently attaching the insulation to a wall, and adds somewhat to the structural integrity of the insulation. Moreover, a skin on the face of insulation exposed to air flow, especially in a duct, can reduce the amount of energy required to force air through the duct. It can also protect against erosion of the insulation thereby reducing or eliminating contamination of flowing air with insulation particles or contamination of the insulation by toxic, corrosive, liquid or solid particles or other undesirable components in the air stream.
In certain applications, the skin can be produced in situ, that is, by depositing a layer of fluid capable of curing to a solid onto a surface of bare insulation material, and then curing the fluid. The fluid can be applied in many ways, for example, by spraying, dipping and roller coating. Coating of insulation material is unique in that the substrate can be semi-rigid or non-rigid, very porous, compressible and it can have a textured surface due to the fibrous nature of the insulation. Additionally, the coating fluid usually is quite viscous. It has been found particularly desirable to utilize a blade coating apparatus to coat insulation material.
Blade coating generally involves placing a thick deposit of coating fluid on the surface of the substrate positioned on one side of a broad, rigid blade. The blade then is dragged across the stationary surface causing the fluid to spread out and form a layer on the surface as it is over run by the blade. For example, U.S. Pat. No. 5,567,504 provides a glass fiber duct board with coated grooves. The patent discloses a method of cutting grooves into a face-coated insulation board, depositing a polymeric coating material into the grooves and then distributing the deposited material over the surfaces of the grooves with a wiper blade to coat the surfaces.
Traditional coating blades have a rectilinear cross section profile (having right-angled, or "square-edged" leading and trailing surfaces) or curvilinear cross section profile (oval, elliptical, circular, etc.). These blades have certain drawbacks. Square-edged boards tend to snag the rough surface of the substrate and can cut and/or tear the insulation. It also produces a coating layer of irregular thickness. This behavior is particularly aggravated when coating grooved insulation boards as described in U.S. Pat. No. 6,000,437. In the cited application, the coating blade is moved across the surface of the grooved board in a direction perpendicular to the grooves. On encountering a groove, the blade tends to sink into the groove and on leaving the groove, to pull on the forward-facing wall of the groove. Not only is this destructive of the insulation but it produces gaps in the coating which prevents formation of a continuous barrier layer of cured coating material that bridges the grooves. Curvilinear cross section coating blades tend to force the coating fluid too far beneath the surface of the insulation material.
It as been discovered that in situ coating of insulation material with a curable fluid can be routinely achieved to produce high quality coated insulation at high productivity by utilizing a spreader blade with a special cross section shape. The cross section is characterized by a composite profile comprising a leading surface that converges toward the substrate surface, and a trailing surface that is parallel to the surface. Accordingly, there is now provided by this invention a method of coating a sheet comprising,
(a) depositing a bead of coating material on a surface of the sheet; PA1 (b) positioning a spreader blade adjacent the sheet; and PA1 (c) moving at least one of the blade and sheet relative to each other parallel to the surface in a machine direction which causes the bead to contact the blade and to flow between the blade and the sheet, thereby spreading the coating material to a layer of uniform thickness on the sheet;
in which the spreader blade has a cross section perpendicular to the surface defining a composite profile comprising a leading surface converging toward the surface and a trailing surface parallel to the surface.